The Acer C720 Chromebook (and newer chromebooks in general) features a "legacy boot" mode that makes it easy to boot Linux and other operating systems. The legacy boot mode is provided by the SeaBIOS payload of coreboot. SeaBIOS behaves like a traditional BIOS that boots into the MBR of a disk, and from there into your standard bootloaders like Syslinux and GRUB.

Enabling SeaBIOS

After changing to developer mode, configure Chrome OS so that you can log in.

To enable the legacy bios:

Open a crosh window with Ctrl+Alt+T.

Open a bash shell with the shell command.

Become superuser with sudo bash

Enable legacy boot with:

# crossystem dev_boot_usb=1 dev_boot_legacy=1

Reboot the machine

You can now start SeaBIOS by pressing Ctrl-L at the white boot splash screen.

Installing Arch Linux

Create a USB drive with the Arch Linux installer. Plug the USB drive into the Chromebook, and start SeaBIOS with Ctrl-L at the white boot splash screen. Press Esc to get a boot menu and select the number corresponding to your USB drive. The Arch Linux installer boot menu should appear. Follow your favorite installation guide.

A few installation notes:

For a 64-bit installation, use the 2013.10.01 ISO, and boot into the x86_64 installer with the mem=1536m kernel option. (You can download torrent on linuxtracker.org)

A fresh DOS partition table on the SSD with one bootable 16GB root partition works. Note that this will wipe Chrome OS.

Use GUID_Partition_Table (GPT) instead of MBR, you can use cgdisk to create partiton. And don't forget a 1-2M size BIOS boot partition for GRUB is needed.

Xorg Video Driver

Use the xf86-video-intel driver.

$ sudo pacman -S xf86-video-intel

Touchpad Kernel Modules

Enabling the touchpad currently requires building a set of patched Haswell Chromebook kernel modules. Fortunately, ChrUbuntu provides a script for automatically building and installing these modules: cros-haswell-modules.sh.

If you want to remove the "Right Click" behavior from the touchpad from the bottom right area (you can still right click with two finger clicks), you should comment out the following section from /etc/X11/xorg.conf.d/50-synaptics.conf

Synaptiks is a touchpad configuration and management tool for KDE. It provides a System Settings module to configure basic and advanced features of the touchpad. Although it is said to be currently unmaintained. and seems to crash under KDE 4.11, it works well with this Chromebook, KDE 4.12.2.
Another untility, kcm_touchpad, does not work at all.

Reboot for the touchpad to become operational.

Touchscreen (C720P model)

If you're using a touchscreen-enabled model (such as the C720P), you may use a modified version of the previous script to install patched modules for the touchscreen as well

Power Key and Lid Switch Handling

Ignore using logind

Out of the box, systemd-logind will catch power key and lid switch events and handle them: it will do a poweroff on a power key press, and a suspend on a lid close. However, this policy might be a bit harsh given that the power key is an ordinary key at the top right of the keyboard that might be pressed accidentally.

To configure logind to ignore power key presses and lid switches, add the lines to logind.conf below.

Then, create the following cros-sound-suspend.sh file. Only the ehci binding/unbinding lines are listed below; see the alternatives linked above for additional sound suspend handling if you experience issues.

Make sure to make the scrip executable:
# chmod +x /usr/lib/systemd/system-sleep/cros-sound-suspend.sh
Then add the following kernel boot parameters. Different combinations have been mentioned, with tpm_tis.force=1 being the most important.

Once you're done configuring xbindkeys, edit your ~/.xinitrc and place

xbindkeys

before the line that starts your window manager or DE.

Audio

Getting alsa to work with the C720 is as simple as creating or editing your /etc/modprobe.d/alsa.conf and adding the line

/etc/modprobe.d/alsa.conf

options snd_hda_intel index=1

Unresolved Issues

The 64-bit installer in Arch Linux installation ISOs newer than 2013.10.01 causes an immediate system reset

Syslinux fails to set the bootable flag with syslinux-install_update -i -a -m. After setting the bootable flag manually in fdisk and installing Syslinux to the MBR with syslinux-install_update -i -m, SeaBIOS boots syslinux, but syslinux then complains about a missing OS. Use GRUB for now.